1. Field of the Invention
The present invention relates to a photosensitive composition showing an increased solubility in an alkaline aqueous solution by an exposure to an IR light. More particularly, the invention relates to a photosensitive composition suitable for use as an image recording layer of a positive-type planographic printing plate precursor, capable of so-called direct printing plate making. That is, the printing plate precursor can be directly converted to a printing plate by scanning with an IR laser in accordance with a digital signal outputted from a computer or the like.
2. Description of the Related Art
There have been used various photosensitive compositions for visible image formation or as a material for a planographic printing plate. Owing to recent remarkable advances in the laser technology for planographic printing, solid-state lasers and semiconductor lasers emitting a light in the near IR to IR region have become easily available with a high output and a compact size. Such lasers are extremely useful as an exposure light source when a printing plate is made directly in accordance with a digital data such as that outputted from a computer.
A positive-type planographic printing plate precursor for IR laser includes a binder resin soluble in an alkaline aqueous solution, an IR dye generating heat by absorption of light etc. as essential components. The IR dye etc. functions, in an unexposed area (image area), as a dissolution inhibitor that substantially reduces the solubility of the binder resin by an interaction with the binder resin. In contrast, in an exposed area (non-image area), the interaction of the IR dye etc. and the binder resin becomes weaker by the generated heat, and the binder resin dissolves in the alkaline developer, thereby a planographic printing plate is formed.
However, in such positive-type planographic printing plate precursor for IR laser, a difference of solubility to the developer between an unexposed area (image area) and an exposed area (non-image area) is still not sufficient under various conditions of use. Accordingly, an excessive development or a deficient development often occurs when the condition of use flucturates.
Such drawback results from a fundamental difference in the printing plate making mechanism between the positive-type planographic printing plate precursor for IR laser and a positive-type planographic printing plate precursor for printing plate making by an exposure with UV. More specifically, the positive-type planographic printing plate precursor for printing plate making by an exposure with UV requires in addition to the binder resin soluble in an alkaline aqueous solution, an onium salt and a quinonediazide as essential components, which perform two functions of serving, in an unexposed area (image area), as a dissolution inhibitor by an interaction with the binder resin, and, in an exposed area (non-image area), being decomposed by light to generate an acid, thereby serving as a dissolution accelerator.
On the other hand, in the positive-type planographic printing plate precursor for IR laser, the IR dye etc. only functions as a dissolution inhibitor in an unexposed area (image area) and does not accelerate the dissolution of the exposed area (non-image area). It is associated with a drawback of often generating a residual film since, in the vicinity of an interface of an exposed area with a substrate, the generated heat tends to diffuse into the substrate and is not efficiently utilized for image formation.
For resolving such drawback, it is proposed to use a binder resin having a higher solubility to the alkaline developer or to add a compound, such as a melamine derivative, having an amino group and showing a higher reactivity. However, such methods makes a state prior to the development unstable, eventually leading to a lowered shelf-stability.
On the other hand, in order to increase a film strength, Japances Patent Application Laid-Open (JP-A) No. 10-282643 discloses an example in which polyacrylic acid is comprised in a positive-type photosensitive composition including a photothermal converting substance and an alkali-soluble resin. However, further improvement in the film strength is required in view of a further rise in the sensitivity of the photosensitive layer. Also JP-A No. 2001-324808 discloses an example in which a methyl methacrylate-(meth)acrylic acid copolymer is comprised in a positive-type photosensitive composition including a photothermal converting substance and a novolac resin. However, it is still insufficient in improving the film strength while maintaining the sensitivity.